An HXYZ-g HR-Fast-MAS probe for Dramatically Improved Biomolecular Structure Determinations Abstract. Liquid state NMR spectroscopy is arguably one of the best tools for structure determination for soluble proteins. The method provides atomic resolution for modest molecular weight proteins and/or their complexes. The method begins to have difficulty when the molecular weight of the system causes slow molecular motion, which in turn increases the linewidth beyond the point of useful resolution. Solid state NMR (ssNMR) methods have progressed remarkably over the past 15 years to permit improved resolution for these conditions, but they still come well short of the goal of liquid-like resolution on biological macromolecules, such as membrane proteins and the fibrils that are central to Alzheimer?s Disease. The ?Holy Grail? in ssNMR would be the ability to successfully utilize the powerful suite of NMR acquisition and automated structure determination protocols developed for solution NMR, which rely on H-detected triple- and quad-resonance H-decoupled schemes (as such generally permit 8 or 30 times higher S/N than direct detection, for C and?5N respectively) with solid samples of 1-10 mg. The main objective of this Phase II application is to complete the development a four-channel multinuclear ssNMR probe (HXYZ) that has the capability of providing C/?N/H correlations under H decoupling utilizing modest (15 kHz) to fast (> 35 kHz) Magic Angle Spinning (MAS) while detecting H. The resulting resolution, particularly with proposed novel pulse sequences, will be close to that of a typical liquid state experiment on proteins. Four-channel multinuclear probes with gradients have been the workhorse in solution NMR for decades, but they have not been available for ssNMR ? they have been perceived to be impractically difficult to design and build. The Phase-I demonstrated feasibility of an H/X/Y/Z narrow-bore (NB) MAS probe based on a novel ?single-coil? rf circuit optimized for H detection and suitable for use at fields from 7-31 T. The Phase-II probe will be compatible with automated sample exchange, pulsed-field gradients (PFG), NB magnets, and novel NB microwave irradiation methods for Dynamic Nuclear Polarization (DNP). Calculations suggest H J-couplings contribute 5-10 Hz to the remaining H line broadening in rigid proteins, and available data suggest the probe-limited resolution (from thermal gradients and magnetism) in commercially available fast-MAS probes has contributed another 6-25 Hz. A 4-channel MAS probe with order- of-magnitude lower thermal gradients that is capable of 2 Hz H resolution on liquids is expected to enable H linewidths below 0.01 ppm on most of the residues in rigid proteins at 900 MHz and above. The novel circuit will also be tunable to virtually all combinations of interest, such as H/C/H/?N, H/P/C/H, H/P/?Li/C, H/?Al/?Si/?O, and H/C/?Si/?Rh, thereby making it also invaluable in such areas as metabolism, neurology, materials science, catalysis, and sustainable energy.